Since they are cold-blooded, tetras rely entirely on water temperature to maintain proper metabolism. This temperature can vary within the range of 75 to 78 degrees Fahrenheit. Outside of this, the fish will become uncomfortable and stressed. This can weaken their immune systems and put them at risk for bacterial infections. Reliable heating will help you avoid this by maintaining a consistent healthy temperature for your tetras. When properly cared for, they can live from 5 to 10 years.
The right heater will have 3 watts for every gallon in your tank (a 10-gallon tank needs a 30-watt heater). All heaters will have a mark showing how high the water level should be. If the water level gets too low, the heater will work harder to heat the water, leaving it susceptible to a breakdown.
Thermostats are notorious for breaking down in aquarium heaters. The thermostat activates the heater according to water temperature. If it goes out, your heater will either be stuck on or off, making the tank as dangerous as one without a heater at all. As such, not only should you invest in the highest quality heater possible, you should have a way to check the temperature besides the heater's thermostat. The thermostat only tells you the temperature the heater is "trying" to reach. Aquarium thermometers are typically cheap and reliable. Find one that's easy to read and check it daily.
No matter what heater you have, it can't do its job without the water circulation from your filter. Circulation keeps different areas of the tank at the same temperature. Familiarize yourself with your filter's instructions regarding how (and how often) to clean it.
Just as important as maintaining consistent water temperature is avoiding rapid changes in temperature. For this reason, always float the bag your new tetras come in on the surface of your aquarium until the water temperature in the bag is the same as in the tank. If your heater breaks or the power goes out, cover the tank with towels and blankets to insulate it and add warm water as needed. If you don't realize there is a problem until the temperature is well below 75 degrees, do not immediately add hot water. Add warm water a little bit at a time to bring the temperature up gradually.
Be aware that temperature is one of many factors to consider. Others are pH levels, shelter, plants, lighting and community. Tetras are less stressed when in groups of at least eight. Dim light also keeps them calm while showing off their colors. Use the light for between 8 and 10 hours daily. Keep the water at a pH of 6.8. Place stones and live plants to give them shelter from direct light. Include plants both floating on the surface and planted in the substrate (the material on the floor of the tank). Some substrates are better for plants than others. Consult with your aquarium shop on all aspects of your tank before purchasing tetras.